A constant warning time device (often referred to as a crossing predictor or a grade crossing predictor in the U.S., or a level crossing predictor in the U.K.) is an electronic device that is connected to the rails of a railroad track and is configured to detect the presence of an approaching train and determine its speed and distance from a crossing (i.e., a location at which the tracks cross a road, sidewalk or other surface used by moving objects). The constant warning time device will use this information to generate a constant warning time signal for controlling a crossing warning device. A crossing warning device is a device that warns of the approach of a train at a crossing, examples of which include crossing gate arms (e.g., the familiar black and white striped wooden arms often found at highway grade crossings to warn motorists of an approaching train), crossing lights (such as the red flashing lights often found at highway grade crossings in conjunction with the crossing gate arms discussed above), and/or crossing bells or other audio alarm devices. Constant warning time devices are often (but not always) configured to activate the crossing warning device at a fixed time (e.g., 30 seconds) prior to an approaching train arriving at a crossing.
Typical constant warning time devices include a transmitter that transmits a signal over a circuit formed by the track's rails and one or more termination shunts positioned at desired approach distances from the transmitter, a receiver that detects one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic that detects the presence of a train and determines its speed and distance from the crossing. The approach distance depends on the maximum allowable speed of a train, the desired warning time, and a safety factor. Preferred embodiments of constant warning time devices generate and transmit a constant current AC signal on said track circuit; constant warning time devices detect a train and determine its distance and speed by measuring impedance changes caused by the train's wheels and axles acting as a shunt across the rails, which effectively shortens the length (and hence lowers the impedance) of the rails in the circuit. Multiple constant warning devices can monitor a given track circuit if each device measures track impedance at a different frequency.
Federal regulations mandate that a constant warning time device be capable of detecting the presence of a train as it approaches a crossing and to activate the crossing warning devices in a timely manner that is suitable for the train speed and its distance from the crossing. In addition, the device must be capable of detecting trains that approach the crossing from both directions of the crossing (e.g., from east to west and from west to east, north to south and south to north, etc.) and from every possible route (i.e., the physical path) through the crossing.
Legacy crossing warning systems are set up to only provide the warnings to oncoming automobile and pedestrian traffic and have very little recording or reporting capability. In the U.S., the Federal Railroad Administration (FRA) mandates annual testing, requiring the railroad's staff to physically run or simulate train movement from all directions and routes. The results of this testing must be submitted to the FRA. This is a heavy burden and expense to the railroads because e.g., it is time consuming and can require running additional locomotive engines to prove the routes and warning times. The burden and expense is exacerbated for more complicated crossing warning systems having switches and multiple routes.
Thus, there is a need and desire for a fast and reliable technique for determining the direction and route of a train traveling along a railroad track so that the information can be used to satisfy regulations such as e.g., the crossing warning time regulations of the FRA.